Radiation therapy (RT) is a mainstay of tumor treatment. Despite technical advances that greatly improve the RT targeting, collateral normal tissue damage and limited antitumor efficacy remain problematic. This is particularly evident when RT involves lung, a very radiosensitive organ. Following RT, an increase in vascular permeability associated with endothelial cell death, vascular leak, inflammation and angiogenesis contributes to pneumonitis. Death of endothelial cells from latent RT damage contributes to the late stages of lung injury, hallmarked by fibrosis and reduced vascular density. There is no FDA approved agent for protection of lung injury from RT. The focus of this proposal is the development of small molecule inhibitors of vascular endothelial protein tyrosine phosphatase (VE-PTP), a protein known to inhibit Angiopoietin (Ang)/Tie2 signaling. VE-PTP is expressed predominantly in endothelial cells. VE-PTP silencing enhances Tie2 activation, promotes endothelial cell survival, and inhibits angiogenesis. Aerpio has developed potent and highly selective small molecule inhibitors of VE-PTP that activate Tie2 signaling irrespective of the presence of its ligands, Angiopoietin 1 and Angiopoietin 2. One of the Aerpio VE-PTP inhibitors, AKB-9778, was shown to enhance the anti-tumor effects of RT in a mouse model of breast cancer; the effect was associated with vascular normalization and enhanced tumor perfusion. We will test the hypotheses that: 1) inhibition of VE-PTP will reduce RT lung injury (Phase 1), and 2) enhance the anti-tumor effects of RT by tumor vascular normalization (Phase 2). Aim 1. Compare the effect of structurally distinct small molecule VE-PTP inhibitors on radiation-induced endothelial cell death in vitro, and evaluate the pharmacokinetics of the best performing compounds. From ~70 compounds with subnanomolar Ki, six unique compounds were identified based on structural diversity and relative VE-PTP inhibitor potency. These compounds will be further evaluated in cultured primary ECs in vitro to compare their effects on RT-induced endothelial cell death. The compound with greatest efficacy and potency for inhibition of RT-induced endothelial cell death will be used in Aim 2. Aim 2. Determine the effect of selected VE-PTP inhibitors on radiation induced lung injury in vivo. Mice will be treated with selected VE-PTP inhibitor or vehicle control and then subjected to a single fraction of 15 Gy to the whole thorax. Outcome variables assessed at 10 and 20 weeks include pulmonary function studies, in vivo imaging using novel NIR markers of blood volume and inflammation, and histologic examination. Successful demonstration of RT protection of normal lung will provide strong rationale to move forward to Phase 2. Lung damage is the focus here, but the results are broadly applicable to many normal tissues. Reduction in damage to normal tissue, while increasing anti-tumor effects of RT, can improve quality of life and prolong survival of hundreds of thousands of patients who are treated with RT.